This invention relates in general to synthesis of critical device parameters and in particular to apparatus and method of synthesizing a critical temperature of a stator vane.
In that fiery inferno known as a gas turbine engine, chemical energy is converted to mechanical energy. Referring to FIG. 1, compressors LPC and HPC convert ambient air to high pressure air. The air is continuously delivered to a combustor COMB which, through the combustion of fuel, raises the temperature and pressure of the air before it is delivered to a first stage high pressure (HP) turbine HPT. The hot, high pressure air is expanded across the HP turbine, producing shaft power to drive the compressors LPC and HPC. The energy remaining is passed across a low pressure turbine LPT which provides output shaft power. Alternatively, the energy can be passed through an exhaust nozzle which provides a "jet" thrust. The engine stations are as follows:
______________________________________ STATION DESCRIPTION ______________________________________ 0 FREE STREAM 1.0 ENGINE INLET 2.0 LOW PRESSURE COMPRESSOR INLET 2.5 HIGH PRESSURE COMPRESSOR INLET 3.0 HIGH PRESSURE COMPRESSOR DIS- CHARGE/COMBUSTOR INLET 4.0 COMBUSTOR DISCHARGE 4.1 1ST STAGE HIGH PRESSURE TURBINE ROTOR INLET 4.5 GAS GENERATOR DISCHARGE/POWER TURBINE INLET 5.0 POWER TURBINE DISCHARGE ______________________________________
The stator vane 72 of the HP turbine is considered one of the more critical components from a thermal stress point of view (see FIG. 2). The selected critical hot spot of the turbine engine is at the first stage turbine vane, trailing edge, pressure side. This spot is designated generally by reference numeral 74. Gases are hottest at the combustor exhaust, reaching as high as 3000.degree. F., under transient operating conditions. These gases heat the outer portion of the vane 72, especially the pressure side, trailing edge. Because the turbine vane 72 is cooled internally by air diverted from the compressor, a temperature gradient results between the inner and outer portions of the vane 72. The higher the temperature gradient, the greater becomes the thermal stress.
These high gradients often make the HP turbine vane the most likely engine component to fail. Failure is primarily due to low cycle fatigue (LCF). Each engine acceleration and deceleration induces a cycle of thermal stress. The graph of FIG. 3 shows a relationship of peak vane temperature versus LCF turbine life. In the engine's maximum power operating range, a small increase of vane temperature can reduce the service life by a substantial amount. In this case, an increase of only 150.degree. F. can reduce the operating life by 2250 cycles.
To prevent turbine damage induced by excessive, prolonged combustor outlet gas temperature, the engine is operated at a turbine peak temperature that is several degrees below the vane's critical life cycle fatigue temperature. Normally, the turbine is protected by the engine control unit (ECU) based on a measured gas temperature (MGT) at the outlet of the HP turbine, station 4.5. With the engine operating at steady-state operating conditions, adequate engine life can be assured by limiting peak temperature based on MGT. During transient operation, however, peak temperature may be exceeded because the response of the MGT is inadequate and does not reflect the true critical turbine temperature.
The MGT does not reflect true critical temperature because the thermocouple probes at the HP turbine exit are constructed for accuracy and durability, not quick response. The MGT thermocouple probe construction results in a lag with third order characteristic, which is very difficult to compensate for with measurements available to the ECU. The result is relatively slow response characteristics as compared to that of the critical first stage turbine hardware. With the engine capable of accelerating from idle to maximum power in just over a couple of seconds, transient gas temperatures quickly increase by more than 2000.degree. F. Thus, thermocouple probe heat transfer characteristics make meaningful signal compensation a difficult task.
Although consideration of this signal lag is not critical for engine accelerations of long duration, the delay becomes most significant when attempting to accurately compensate for thermocouple dynamics during rapid accelerations of short duration.